Marmot - Trestles 0

If you only plan to car camp, don't bother with the down sleeping bags because the main benefits of down (compressible, lightweight) aren't really important features for car camping. On the other hand, the main benefit of synthetic insulation is that it still works if you get it wet, and it is cheaper. I prefer synthetic insulation for car camping.  I had a Marmot Trestles 0 degree for years and used it when I was a trail worker camping 7 nights in a row and it never let me down. Big and heavy for carrying in a backpack, but supremely warm. Sleeping 2 people in a car, the 15 degree would probably be great. You can always bring some extra blankets to supplement the sleeping bags.  I do a lot of trips where I sleep in the back of my vehicle. I've also done a lot of backpacking trips where I carry all of my stuff and sleep on a combo lightweight air mattress and foam pad. Take this advice that I learned the hard way - skip the air mattress for car camping. Air does not provide insulation, and usually will wind up having weird gaps between the mattress and the vehicle. Sounds like you already have a mattress topper, but maybe there is one that's a better fit in your vehicle? Foam is way warmer than air.  Also, maybe consider a layer or two of a more dense eva foam to go under your mattress topper. Sold as anti-fatigue mats, and come as single pieces or 2 foot by 2 foot puzzle pieces. Put that down under your mattress topper and your bed will be comfy, warm, and un-poppable. We roll up our mattress topper & bedding during the day and leave the anti fatigue mat rolled out with our stuff on it. We have [two of these](https://www.homedepot.com/p/TrafficMaster-Diamond-Soft-Foot-Black-4-ft-x-6-ft-Indoor-Outdoor-Commercial-Door-Mat-6016909004x6/306932662). Trim to size as needed.  Another hot take - don't build a platform other than a basic low profile sheet if you need it. In my opinion they wind up adding weight and taking up all of the space inside and leave you sleeping in a coffin. For a smaller vehicle like a highlander you're better off having your stuff well packed and just moving some stuff to the front seats or outside while you sleep. Being able to sit up in the middle of the night is priceless to me so I'll take the 5 minutes of shuffling stuff to the front if it means I don't get claustrophobic at night.  I've had a lot of great nights out camping in my vehicle and I love it. I really hope this helps, and hope you can create a setup that works well and comes in under budget! 

Not sure if weight matters, I have an older version of the marmot trestles 0°f/ -18°c synthetic fill. I find it to be a very warm bag. I have been comfortable in that bag sleeping in my jeep on a cold Vermont night. It’s $200 usd not exactly the most packable or lightest bag I own but it’s def warm.

Op I would caution against marmot I have a marmot long mummy style bag I’m also 6’6” and it’s way to tight in the hips and shoulders (I can’t even zip it up I gave it to one of the kids and am happy I bought it cheap used lol) like you I’m currently looking at a Teton

DO NOT BUY A KIDS’ BAG. They are not rated accurately in comparison to adult bags. I’m 110lbs/5’3” and I brought a kid 0F bag on the John Muir Trail. Learned the hard way.  I have a 0F Marmot that I love.

Love my Marmot Trestles!

Marmot Trestles line are good deals on good bags. If you’re younger, just get a blue foam pad.